1. Socket Interface 1 Introduction 11 Socket address formats 2 API 12 13
listen 13
accept 3
The Socket API 14
connect 4
Sockets and socket libraries 15
send 5
Sockets and UNIX I/O 16
sendto, sendmsg 6
The socket API 17
recv 7
Summary of socket system calls 18
recvfrom, recvmsg 8
socket 19
Other procedures 9
close 20
Sockets and processes 10
bind 21
Summary

2. Introduction
The socket is one form of interface between application programs and protocol software
Widely available - program portability
Used by both clients and servers
Extension to UNIX file I/O paradigm

3. API Application interactions with protocol software:
Passive listen or active open
Protocol to use
IP address and port number
Interface to protocol is call Application Program Interface (API)
Defined by programming/operating system
Includes collection of procedures for application program

4. The Socket API Protocols do not typically specify API
API defined by programming system
Allows greatest flexibility - compatibility with different programming systems
Socket API is a specific protocol API
Originated with Berkeley BSD UNIX
Now available on Windows 95 and Windows NT, Solaris, etc.
Not defined as TCP/IP standard; de facto standard

5. Sockets and socket libraries
BSD UNIX includes sockets as system calls
Other vendors (mostly UNIX) have followed suit
Some systems have different API
Adding sockets would require changing OS
Added library procedures - socket library - instead
Adds layer of software between application and operating system
Enhances portability
May hide native API altogether

6. Sockets and UNIX I/O Developed as extension to UNIX I/O system
Uses same file descriptor address space (small integers)
Based on open-read-write-close paradigm
open - prepare a file for access
read/write - access contents of file
close - gracefully terminate use of file
Open returns a file descriptor, which is used to identify the file to read/write/close

7. The socket API Socket programming more complex than file I/O
Requires more parameters
Addresses
Protocol port numbers
Type of protocol
New semantics
Two techniques
Add parameters to existing I/O system calls
Create new system calls
Sockets use a collection of new system calls

8. Summary of socket system calls
socket - create a new socket
close - terminate use of a socket
bind - attach a network address to a socket
listen - wait for incoming messages
accept - begin using incoming connection
connect - make connection to remote host
send - transmit data through active connection
recv - receive data through active connection

9. descriptor = socket(protofamily, type, protocol)
Returns socket descriptor used in subsequent calls
protofamily selects protocol family; e.g.:
PF_INET - Internet protocols
PF_APPLETALK - AppleTalk protocols
type selects type of communication
SOCK_DGRAM - connectionless
SOCK_STREAM - connection-oriented
protocol specifies protocol within protocol family
IPPROTO_TCP - selects TCP
IPPROTO_UDP - selects UDP

10. close close(descriptor) Terminates use of socket descriptor
descriptor contains descriptor of socket to be closed

11. bind(socket, localaddr, address)
Initially, socket has no addresses attached
bind selects either local, remote or both addresses
server binds local port number for incoming messages
client binds remote address and port number to contact server

12. Socket address formats
Because sockets can be used for any protocols, address format is generic:
struct sockaddr {
u_char sa_len; /* total length of address */
u_char sa_family; /* family of the address */
char sa_data[14]; /* address */ }
For IP protocols, sa_data hold IP address and port number:
struct sockaddr_in {
u_char sin_len; /* total length of address */
u_char sin_family; /* family of the address */
u_short sin_port; /* protocol port number */
struct in_addr sin_addr; /* IP address */
char sin_zero[8] /* unused */
First two fields match generic sockaddr structure
Remainder are specific to IP protocols
INADDR_ANY interpreted to mean "any" IP address
Socket address formats

13. listen(socket, queuesize)
Server uses listen to wait for incoming connections
socket identifies socket through which connections will arrive (address)
New connection requests may arrive while server processes previous request
Operating system can hold requests on queue
queuesize sets upper limit on outstanding requests

14. accept(socket, caddress, caddresslen)
Server uses accept to accept the next connection request
accept call blocks until connection request arrives
Returns new socket with server's end of new connection
Old socket remains unchanged and continues to field incoming requests
caddress returns struct sockaddr client address; format depends on address family of socket
caddresslen returns length of address

15. connect(socket, saddress, saddresslen)
Client uses connect to establish connection to server
Blocks until connection completed (accepted)
socket holds descriptor of socket to use
saddress is a struct sockaddr that identifies server
saddresslen gives length of saddress
Usually used with connection-oriented transport protocol
Can be used with connectionless protocol
Marks local socket with server address
Implicitly identifies server for subsequent messages

16. send(socket, data, length, flags)
Used to send data through a connected socket
socket identifies socket
data points to data to be sent
length gives length of data (in bytes)
flags indicate special options

17. sendto(socket, data, length, flags, destaddress, addresslen)
sendmsg(socket, msgstruct, flags)
Used for unconnected sockets by explicitly specifying destination
sendto adds additional parameters:
destaddress - struct sockaddr destination address
addresslen - length of destaddress
sendmsg combines list of parameters into single structure:
struct msgstruct {
struct sockaddr *m_addr; /* ptr to destination address */
struct datavec *m_vec; /* pointer to message vector */
int m_dvlength; /* num. of items in vector */
struct access *m_rights; /* ptr to access rights list */
int m_alength; /* num. of items in list */ }
sendto, sendmsg

18. recv(socket, buffer, length, flags)
Used to receive incoming data through connected socket
socket identifies the socket
Data copied into buffer
At most length bytes will be recved
flags give special options
Returns number of bytes actually recved
0 implies connection closed
-1 implies error

19. recvfrom, recvmsg
recvfrom(socket, buffer, length, flags, sndraddress, addresslen) recvmsg(socket, msgstruct, flags)
Like sendto and sendmsg (in reverse!)
Address of source copied into sndraddress
Length of address in addresslen
recvmsg uses msgstruct for parameters

20. Other procedures getpeername - address of other end of connection
getsockname - current address bound to socket
setsockopt - set socket options

21. Sockets and processes
Like file descriptors, sockets are inherited by child processes
Socket disappears when all processes have closed it
Servers use socket inheritance to pass incoming connections to slave server processes

22. Summary Socket API is de facto standard
Originally developed for BSD UNIX
Copied to many other systems
Sockets are an extension of the UNIX file I/O system
Use same descriptor addresses
Can (but typically don't) use same system calls
Many specific system calls for sockets